The present invention relates to a method and a device for monitoring a spatial region, especially in the context of safeguarding a hazardous machine.
EP 1 543 270 B1 discloses a method and a device which serve for safeguarding the hazardous region of a machine operating in automated fashion. The device has at least two image recording units arranged at a defined distance from one another. Each image recording unit provides an image of the spatial region in which the machine is arranged. The images of the two image recording units are evaluated by means of at least two different scene analysis methods, in order to obtain a three-dimensional image of the monitored spatial region. In other words, positions of individual objects in the spatial region are determined on the basis of the images recorded in parallel, wherein said positions also include distance information. Afterward, on the basis of defined protection regions or protection spaces, it is possible to decide whether an object, such as a person, for instance, has approached the machine operating in automated fashion to an extent such that a hazard exists. If appropriate, the machine is switched off or operated at reduced speed.
With such a device, the scene analysis methods and, in particular, the determination of object distances are typically based on objects and/or structures in each of the two recorded images, which objects have to be identified and assigned to one another. Since the two image recording units are arranged at a distance from one another, each image recording unit has a different viewing angle. Therefore, identical objects and/or structures appear offset with respect to one another in the two images, wherein the relative offset of an object in the two images is dependent on the defined distance between the image recording units and on the distance of the object with respect to the image recording units. Consequently, given a known distance between the image recording units, it is possible to determine the distance to the object on the basis of the images. The functional principle corresponds in a certain way to human three-dimensional vision.
The Assignee's practical experience with a device of this type has shown that a reliable and accurate distance measurement using this principle is very complex and difficult if structures having a plurality of relatively closely spaced substantially parallel edges are present within the monitored spatial region. Such edges can be three-dimensional structures such as, for instance, a staircase, a lattice fence or vertical blinds, or they can be two-dimensional structures such as, for instance, a striped pattern on the floor or on a wall. The parallel edges make it more difficult to unambiguously assign an edge in the two recorded images. An incorrect assignment, where different edges are assigned to one another, generally leads to an incorrect distance measurement. This is particularly problematic if the device and the method are used for safeguarding a hazardous region, such as, for instance, the hazardous region of a machine operating in automated fashion.
DE 10 2005 063 217 A1 describes a method for configuring a prior art device for monitoring a spatial region. The configuration includes the definition of protection spaces and/or protection regions on the basis of variable geometry elements. In one exemplary embodiment, the geometry elements are produced graphically in or above a real image of the spatial region to be monitored. The real image can include a number of reference marks that are used to define a configuration plane. The variable geometry element is then produced relative to the configuration plane in order to provide an unambiguous relationship between the “virtual” geometry element and the real spatial region. Incorrect assignments of parallel edges and measurement errors ensuing therefrom cannot, however, be avoided with the known method.